A turbofan engine used as an engine of an aircraft comprises a fan that produces most of the thrust and a core engine (turbojet engine) provided with a turbine that is disposed behind the fan and drive the fan. The fan and the core engine are coaxially arranged, air sucked by the fan from the front of the engine is divided into air (Gf) that passes through the fan and is discharged to the rear and air (Gc) that is introduced into the core engine, used for combustion to make the turbine rotate and then discharged to the rear. The ratio between the two flows of air (Gf/Gc) is referred to as a bypass ratio.
The higher the bypass ratio, the higher the fuel efficiency is, so that turbofan engines of high bypass ratios have been developed in recent years. As the bypass ratio increases, the ratio of the diameter of the fan to the diameter of the core engine increases, so that the length of the fan blades forming the fan increases. Although conventional fan blades are made of titanium, an aluminum alloy or the like, it has been proposed to use a fiber-reinforced composite material for the fan blades in order to reduce weight and ensure strength in the trend toward larger fan blades (see Patent Documents 1 and 2).
Such a fan blade made of a composite material is molded by thermoforming from a blank sheet that comprises a plurality of fibers arranged in parallel with each other (filaments) consolidated by a resin (a polymer). To mold a product having a three-dimensional shape, such as the fan blade, thermoforming from a blank sheet having a flat shape, the blank sheet is heated to make the resin soften, and then (a) the blank sheet is sandwiched between two molds, (b) the blank sheet is pressed against a mold by compressed air, or (c) the space between a mold and the blank sheet is decompressed to make the blank sheet cling to the mold (see Patent Documents 3 and 4).